Space Time Block Code Communications with Co-Operative Relays

ABSTRACT

Methods, systems and apparatuses are provided for transmitting and receiving space-time block coded data in a wireless communications system with co-operative relays. A source node transmits RF signals representing first and second sets of data symbols in respective first and second channels (in time frequency code or any combination) of a wireless communications system, the first and second sets of data symbols being for transmission from separate antennas respectively according to a space-time block code. A relay node receives the RF signals representing the first set of data symbols in the first channel and transmits RF signals representing the first set of data symbols in the second channel. A destination node received the RF signals representing the second set of data symbols from the source node and the RF signals representing the first set of data symbols from the relay node. This enables decoding of the received RF signals representing the first and second sets of data symbols according to the space-time block code.

FIELD OF THE INVENTION

The present invention relates to methods, systems and apparatuses fortransmitting and receiving space-time block coded data in a wirelesscommunications system with co-operative relays.

BACKGROUND OF THE INVENTION

In a conventional cellular wireless system, a base station communicatesdirectly with a user terminal. If two antennas are available at the basestation, a space time block code (STBC), such as an Alamouti STBC, maybe used to obtain diversity gain, thus improving the quality of thereceived signal. A multihop network differs from a conventional cellularnetwork in that one or more relays may be included in the path between abase station and a user terminal. Multihop diversity, sometimes known ascooperative relaying, combines the signals from the source (e.g., basestation) and a relay at the destination (e.g., user terminal) to improvethe quality of the received signal. The purpose of a multihop system isto enhance the signal quality at the destination (base station on theuplink and user terminal on the downlink) in comparison with aconventional cellular wireless system.

A number of alternative cooperative relaying schemes have been proposedbut these involve multiple relays acting in parallel in each path, whichmeans more relays are required, resulting in a higher network cost.Space time coding has also been proposed for use with multihop systemsbut schemes known to the author involve multiple antennas at the basestation and the relays, and sometimes at the user terminal as well.While the use of multiple antennas at the base station is practical,multiple antennas at the relay would mitigate against many deploymentoptions which are attractive for other reasons (e.g., cost ofdeployment, access to sites, etc.). The use of multiple antennas at theuser terminal is something that terminal manufacturers have resisted intheir efforts to reduce cost and size of the terminal.

SUMMARY OF THE INVENTION

This invention describes a method of employing STBCs, such as AlamoutiSTBCs, in combination with cooperative relaying in a multihop network tofurther improve the quality of the received signal. This inventionprovides for a further enhancement of the received signal quality in anefficient manner requiring minimum complexity in the respective nodes.This means that fewer relays are required for a given performance, thusreducing the overall cost of the network. Only one antenna is requiredat each node (i.e., base station, relay and user terminal) althoughextension to multiple antennas is possible. The invention also describesan efficient method of arranging the transmissions of the various nodes(base station, relay and user terminal) to avoid violating causality andto provide adequate time for each node to process the received signalsin an efficient manner.

According to a first aspect of the present invention there is provided amethod of transmitting data using a space-time block code in a wirelesscommunications system comprising a source node and a relay node, themethod comprising:

-   -   the source node transmitting RF signals representing first and        second sets of data symbols in respective first and second        channels of the wireless communications system, the first and        second sets of data symbols being for transmission from separate        antennas respectively according to the space-time block code;        and    -   the relay node receiving the RF signals representing the first        set of data symbols in the first channel and transmitting RF        signals representing the first set of data symbols in the second        channel.

According to a second aspect of the present invention there is provideda wireless communications system using a space-time block code fortransmissions, the system comprising:

-   -   a source node arranged to transmit RF signals representing first        and second sets of data symbols in respective first and second        channels of the wireless communications system, the first and        second sets of data symbols being for transmission from separate        antennas respectively according to the space-time block code;        and    -   a relay node arranged to receive the RF signals representing the        first set of data symbols in the first channel and to transmit        RF signals representing the first set of data symbols in the        second channel.

According to a third aspect of the present invention there is provided amethod of transmitting data using a space-time block code in a wirelesscommunications system comprising a source node, the method comprising:

-   -   the source node transmitting RF signals representing first and        second sets of data symbols in respective first and second        channels of the wireless communications system, the first and        second sets of data symbols being for transmission from separate        antennas respectively according to the space-time block code.

According to a fourth aspect of the present invention there is provideda source node for use in a wireless communications system fortransmitting data using a space-time block code, the source node beingarranged to transmit RF signals representing first and second sets ofdata symbols in respective first and second channels of the wirelesscommunications system, the first and second sets of data symbols beingfor transmission from separate antennas respectively according to thespace-time block code.

According to a fifth aspect of the present invention there is provided amethod of transmitting data using a space-time block code in a wirelesscommunications system comprising a relay node, the method comprising:

-   -   the relay node receiving RF signals representing a first set of        data symbols in a first channel of the wireless communications        system; and    -   the relay node transmitting RF signals representing the first        set of data symbols in a second channel of the wireless        communications system;    -   wherein the second channel of the wireless communications system        is for transmission of a second set of data symbols, the first        and second sets of data symbols being for transmission from        separate antennas respectively according to the space-time block        code.

According to a sixth aspect of the present invention there is provided arelay node for use in a wireless communications system for transmittingdata using a space-time block code, the relay node being arranged toreceive RF signals representing a first set of data symbols in a firstchannel of the wireless communications system and to transmit RF signalsrepresenting the first set of data symbols in a second channel of thewireless communications system,

-   -   wherein the second channel of the wireless communications system        is for transmission of a second set of data symbols, the first        and second sets of data symbols being for transmission from        separate antennas respectively according to the space-time block        code.

The invention to be described herein affords several advantages. Thebase station (BS) only needs to transmit on the “downlink” channel andreceive on the “uplink” channel. The relay is only required to transmiton the “downlink” channel and to receive on the “uplink” channel,although it may need to do so simultaneously in some embodiments in anFDD system. The RN (relay node) is therefore very simple and potentiallyinexpensive. The user terminal (UT) is only required to transmit on the“uplink” channel and to receive on the “downlink” channel, although itmay need to do so simultaneously in some embodiments in an FDD system.The most significant modification in practical products is to enable theUT and BS to decode Alamouti STBCs and optionally to store andsubsequently combine signals from the BS and UT respectively with thosefrom the RN, providing additional diversity; however, this is optionaland so UTs that do not have this capability are not excluded from thesystem.

There now follows, by way of example only, a detailed description ofpreferred embodiments of the present invention in which:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows transmission on the downlink in accordance with the presentinvention;

FIG. 2 shows transmission on the uplink in accordance with the presentinvention;

FIG. 3 shows timeslot usage in uplink and downlink in accordance withthe present invention;

FIG. 4 shows a frame structure for staggering uplink and downlinktransmission in accordance with the present invention;

FIG. 5 shows a scenario for interleaving transmissions to two userterminals in accordance with the present invention;

FIG. 6 shows downlink transmissions in the scenario for interleavingtransmissions to two user terminals in accordance with the presentinvention; and

FIG. 7 shows a graph of a distribution of path lengths for routing, pathselection and scheduling in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The following is a description of the best mode of practicing theinvention known to the inventor.

FIG. 1 shows transmission on the downlink in accordance with the presentinvention. A base station BS 10 transmits a downlink signal which isreceived by a relay node RN 12 as downlink signal 20 and by a userterminal UT 14 as downlink signal 24. RN 12 also transmits a downlinksignal 22 which is received by UT 14. Tables 30, 32, and 34 show thecontents of downlink signals 20, 22 and 24 respectively. Each signalcomprises two downlink channels—timeslot TS₁ and timeslot TS₂.

The contents of downlink signals 20 and 24 transmitted from BS 10comprise two symbols (s₁, s₂) in TS₁ and a function of the two symbols(−s₂*, s₁*) in TS₂ (where x* represents the complex conjugate of x). Ifthese symbols (s₁, s₂) and (−s₂*, s₁*) were transmitted in the samedownlink channel (e.g., in TS₁) from two separate antennas of the basestation for reception at a single antenna of a user terminal they wouldtogether form an Alamouti STBC as known in the art. However, in thepresent invention, they are transmitted using two separate downlinkchannels—timeslot TS₁ and timeslot TS₂, and optionally from a singleantenna of the base station. Note the contents of TS₂ are “greyed out”in table 30 to indicate that although the signal is transmitted from BS10 it is not necessarily received by RN 12. The RN typically cannotreceive in TS₂ because according to the present invention it istransmitting on the same channel. The contents of downlink signal 22transmitted from RN 12 comprise (s₁, s₂) in TS₂—i.e. the same as thecontents of TS₁ in downlink signal 20. Thus, RN 12 stores the signal(s₁, s₂) received from BS 10 on TS₁ for one TS and transmits it to UT 14in TS₂.

The BS and RN transmissions 22 and 24 received at UT 14 in TS₂ togetherform an Alamouti STBC which is decoded at the UT. The extra informationcontained in TS₁ from BS 10 enables 3-branch diversity at the UT. The UTstores it for one TS and combines it with the Alamouti code. This may bedone through i) addition of the received RF signals at baseband, ii)apportionment of the received RF signals at baseband, iii) selectionbetween the received RF signals at baseband according to a measure ofsignal quality; iv) demodulation of the received RF signals intodemodulated RF signals followed by combination of the demodulated RFsignals (by simple addition thereof, apportionment thereof or selectionthere between), or by demodulation and decoding of the received RFsignals into data symbols followed by combination of the decoded datasymbols. In case the UT cannot decode Alamouti STBCs, then the BS willnot transmit in TS₂.

FIG. 2 shows transmission on the uplink in accordance with the presentinvention. UT 14 transmits an uplink signal which is received by RN 12as uplink signal 42 and by BS 10 as uplink signal 44. RN 12 alsotransmits an uplink signal 40 which is received by UT 14. Tables 50, 52,and 54 show the contents of uplink signals 40, 42 and 44 respectively.Each signal comprises two uplink channels—timeslot TS₁ and timeslot TS₂.

The contents of uplink signals 42 and 44 transmitted from UT 14 comprisetwo symbols (s₁, s₂) in TS₁ and a function of the two symbols (−s₂*,s₁*) in TS₂ (where x* represents the complex conjugate of x). If thesesymbols (s₁, s₂) and (−s₂*, s₁*) were transmitted in the same uplinkchannel from two separate antennas of the user terminal for reception ata single antenna of a base station, they would together form an AlamoutiSTBC as known in the art. However, in the present invention, they aretransmitted using two separate uplink channels—timeslot TS₁ and timeslotTS₂, and optionally from a single antenna of the user terminal. Note thecontents of TS₂ are “greyed out” in table 42 to indicate that althoughthe signal is transmitted from UT 14 it is not necessarily received byRN 12. The RN typically cannot receive in TS₂ because according to thepresent invention it is transmitting on the same channel. The contentsof uplink signal 40 transmitted from RN 12 comprise (s₁, s₂) in TS₂—i.e.the same as contents of TS₁ in uplink signal 42. Thus, RN 12 stores thesignal (s₁, s₂) received from UT 14 on TS₁ for one TS and transmits itto BS 10 on TS₂.

The UT and RN transmissions 44 and 40 received at BS 10 in TS₂ form anAlamouti STBC which is decoded at the UT. The extra informationcontained in TS₁ from UT 14 enables 3-branch diversity at the BS. The BSstores it for one TS and combines it with the Alamouti code. This may bedone through i) addition of the received RF signals at baseband, ii)apportionment of the received RF signals at baseband, iii) selectionbetween the received RF signals at baseband according to a measure ofsignal quality; iv) demodulation of the received RF signals intodemodulated RF signals followed by combination of the demodulated RFsignals (by simple addition thereof, apportionment thereof or selectionthere between), or by demodulation and decoding of the received RFsignals into data symbols followed by combination of the decoded datasymbols.

Note in the above, downlink is BS to UT, uplink is UT to BS. Downlinkand uplink channels may be different frequencies in a Frequency DivisionDuplex system (FDD) or time periods in a Time Division Duplex system(TDD). While embodiments described in this document have shown howuplink and downlink channels are provided using two timeslots orfrequencies each having two symbols to provide space time block codingin a co-operative relaying scheme, it will be apparent to one skilled inthe art that any orthogonal channels may be used for each set of symbolsprovided that the channel on which the relay node transmits a set ofsymbols is later in time than the channel on which it receives that setof symbols. For example, channels may be orthogonal in time, frequency,spreading code or any combination of the same provided that the channelon which the relay node transmits a set of symbols is later in time thanthe channel on which it receives that set of symbols.

Also note in the above that the function of the RN can be asrepeater—i.e. a node which receives an RF downlink or uplink signal andre-transmits it without demodulation, decoding or other processing—or asa regenerator—i.e. a node which receives an RF signal, performsdemodulation, decoding and/or some other digital processing on the RFsignal to determine the transmitted data (after conversion to digital)and then re-generates a new RF signal using the appropriate modulationand coding using the determined data. In either case, as describedabove, the RN transmits a downlink or uplink signal representing thesame data as it has received.

As can be seen from the description of downlink and uplink transmissionschemes above, the BS is not significantly changed. The BS transmits onthe downlink channel and receives on the uplink channel. However, the BSmust store the signal received from the UT in TS₁ for one TS and combineit with the signal received from the RN in TS₂ as well as decoding theSTBC in order to gain full advantage of the available diversity.

The UT is relatively simple. According to the downlink and uplinktransmission schemes above, the UT transmits on the uplink channel, asit would for a direct transmission to a BS. It receives on the downlinkchannel, as it would for direct reception from a BS. The UT must be ableto decode Alamouti STBCs in order to obtain the additional diversity,but this is optional. It may also combine signals from both timeslotsfor extra diversity if memory is available.

The RN is more complex. According to the downlink and uplinktransmission scheme above, the RN must transmit on both the downlink anduplink channels. The RN must receive on both the downlink and uplinkchannels. It may need to transmit and receive simultaneously.

FIG. 3 shows timeslot usage in uplink and downlink in accordance withthe present invention. Four timeslots are available: TS₁ and TS₂ on thedownlink channel; TS₁ and TS₂ on the uplink channel. Two timeslots worthof data are received: one by the UT on the downlink (downlink channelTS₂); one by the BS on the uplink (uplink channel TS₂). Efficiency istherefore 2/4=½. The advantage is that both the UT and the BS arerelatively unchanged from a conventional system. Only the RN needs totransmit and receive on both the uplink and the downlink channels. TheRN is transmitting on the uplink and downlink channels in the same TS,namely TS2, (i.e., simultaneously in a FDD system. In a TDD system, TS1and TS2 of the downlink channel would be separated in time from TS1 andTS2 of the uplink channel).

To avoid this problem, we optionally slip the uplink relative to thedownlink by one TS, which has a limited effect on efficiency if theframe length is many TSs (as shown in FIG. 4), or use different RNs foruplink and downlink. Staggering uplink and downlink timeslots means thata RN does not have to transmit on both uplink and downlink channels atthe same time in an FDD system. The efficiency is degraded slightly bythis slippage in the uplink TS:

Efficiency=L/L+1

FIG. 5 shows a scenario for interleaving transmissions to two userterminals in accordance with the present invention and FIG. 6 showsdownlink transmissions in the scenario for interleaving transmissions totwo user terminals in accordance with the present invention. Accordingto the downlink and uplink transmission scheme above, the RN is requiredto receive on TS₁ and transmit on TS₂. This may be difficult to achievein practice in a digital system as it leaves no time for processing thereceived signal. To overcome this problem, transmissions from BS 68 totwo (or more) UTs 60, 62 using two (or more) RNs 64, 66 are interleavedso that there is a one TS gap of between reception and transmission ateach RN (see FIG. 5 and FIG. 6).

As stated above, multihop diversity, or cooperative relaying, combinesthe signals from a source (e.g., base station) and a relay at adestination (e.g., user terminal). It will be apparent to one skilled inthe art that while, in two hop scenarios, the source and destination maybe a base station or user terminal, in three or more hop scenarios, thesource and/or the destination may themselves be further relay nodesbetween a base station and user terminal. Thus, it will be appreciatedthat the above description of uplink and downlink transmission schemesutilizing STBC and multihop diversity according to the present inventionapplies generally to transmissions from any source (on the downlink abase station or a relay node, and on the uplink a relay node or userterminal) to any destination (correspondingly on the downlink a userterminal or relay node, and on the uplink a relay node or base station)using a relay node.

Different STBCs may be used for uplink or downlink communications andfor different two-hop stages of the uplink and downlink in three or morehop scenarios. For example, the STBC used for downlink by the basestation may be different to the STBC used for uplink by the userterminal.

FIG. 7 shows a graph of an example of a distribution of path lengths forrouting, path selection and scheduling in accordance with the presentinvention (0 represents UTs out of coverage; 1 is the direct pathbetween the BS and the UT). Routing algorithms are generally too slow toreact to Rayleigh fading and so path selection must be based on pathloss due to distance and lognormal shadowing. Advantage may be taken ofmulti-user diversity by scheduling to each user depending on the totalpath loss, including Rayleigh fading, at any given time; however, thenumber of simultaneous users may be restricted at the high data ratesfor which use of multihop is proposed as a coverage enhancementmechanism, thus restricting the diversity benefit.

It is proposed that greater diversity benefit can be obtained bycombining multihop routing and scheduling as follows: For each UT,several multihop paths are found and stored (instead of just one) andthese are updated on a timescale determined by the change in averagepath loss, which is predominantly due to changes in lognormal shadowing.Packets may be scheduled across any one of these paths to any one of theseveral users determined by the combination of the path loss due todistance, lognormal shadowing and Rayleigh fading on each path. Thus thedegree of diversity is increased and advantage can be taken of theRayleigh up fades to improve throughput.

Multihop paths should only be used if the existing direct path isincapable of providing sufficient signal quality; furthermore, longermultihop paths (>2 hops) should only be used if shorter paths areincapable of providing sufficient signal quality. What is deemedsufficient may be determined by the desire to maximise coverage orcapacity or minimise delay, etc. We therefore propose that a qualitymetric is fed back from the destination into the path selectionalgorithm, whether this is centralised (source routed) or distributedand that this be used to optimise path selection. Our preferred metricis SINR at the destination, though other metrics can be envisaged.

Multihop paths provide higher SINRs than direct paths in many instancesand so a multihop network can provide better coverage than aconventional cellular network. However, longer multihop paths increaselatency and multihop proposals often restrict path lengths to 2 hops. Wenote, however, that where longer paths are allowed, they are only used asmall percentage of the time (FIG. 7) and that the higher SINR providedby them results in a higher bit rate to further reduce the timerequired. We therefore propose a scheduler which is sensitive to pathlength such that it gives a higher priority to longer paths, thusreducing the combined scheduling delay on such paths and hence reducingthe delay spread in a multihop network. Because the longer paths areonly used occasionally (FIG. 7), the overall increase in average delayis small and, consequently, so is the impact on the direct and shorterpaths.

1-40. (canceled)
 41. A method of receiving data in a wirelesscommunications system comprising a source node, a relay node and adestination node, wherein the source node, the relay node, and thedestination node communicate using at least a first channel and a secondchannel, the method comprising: the destination node receiving first RFsignals from the relay node in the second channel, wherein the first RFsignals represent a first set of data symbols; and the destination nodereceiving second RF signals from the source node in the second channel,wherein the data symbols in the second set of data symbols are functionsof the data symbols in the first set of data symbols; wherein the relaynode transmitted the first RF signals in the second channel in responseto receiving third RF signals representing the first set of data symbolsin the first channel; the destination node decoding data represented bythe first and second sets of data symbols using the first and second RFsignals.
 42. The method of claim 41, further comprising: the destinationnode receiving the third RF signals representing the first set of datasymbols from the source node; wherein said decoding the data also usesthe third RF signals.
 43. The method of claim 41, wherein said receivingthe first RF signals and said receiving the second RF signals areperformed using a single antenna.
 44. The method of claim 41, whereinthe first channel and the second channel are different with respect totime.
 45. The method of claim 41, wherein the first channel and thesecond channel are different with respect to frequency.
 46. The methodof claim 41, wherein the first channel and the second channel aredifferent with respect to spreading codes.
 47. The method of claim 41,wherein the relay node receives the third RF signals from the sourcenode and wherein the source node transmits the second RF signals and thethird RF signals from a single antenna.
 48. The method of claim 41,wherein the first and second sets of data together form an Alamoutispace-time block code.
 49. A user terminal for use in a wirelesscommunication system, the wireless communication system comprising asource node and a relay node, the user terminal comprising: at least oneantenna; wireless communication circuitry coupled to the at least oneantenna, wherein the wireless communication is configured to use the atleast one antenna to perform wireless communication; and processinghardware coupled to the wireless communication circuitry, wherein theprocessing hardware is configured to operate with the wirelesscommunication circuitry and at least one antenna to: receive first RFsignals from the relay node in the second channel, wherein the first RFsignals represent a first set of data symbols; and receive second RFsignals from the source node in the second channel, wherein the datasymbols in the second set of data symbols are functions of the datasymbols in the first set of data symbols; wherein the relay nodetransmitted the first RF signals in the second channel in response toreceiving third RF signals representing the first set of data symbols inthe first channel; decode data represented by the first and second setsof data symbols using the first and second RF signals.
 50. The userterminal of claim 49, wherein the processing hardware is furtherconfigure to: receive the third RF signals representing the first set ofdata symbols from the source node; wherein said decoding the data alsouses the third RF signals.
 51. The user terminal of claim 49, whereinsaid receiving the first RF signals and said receiving the second RFsignals are performed using a single antenna of the at least oneantenna.
 52. The user terminal of claim 55, wherein the first and secondchannels are first and second timeslots of a frame structure of thewireless communications system.
 53. The user terminal of claim 55,wherein the first and second channels are first and second frequenciesof a frame structure of the wireless communications system.
 54. The userterminal of claim 55, wherein the first and second channels are firstand second spreading codes of a frame structure of the wirelesscommunications system.
 55. The user terminal of claim 49, wherein therelay node receives the third RF signals from the source node andwherein the source node transmits the second RF signals and the third RFsignals from a single antenna.
 56. The user terminal of claim 49,wherein the first and second sets of data together form an Alamoutispace-time block code.
 57. A relay node for use in a wirelesscommunications system, the wireless communications system comprising asource node and a destination node, wherein the relay node comprises: atleast one antenna; wireless communication circuitry coupled to the atleast one antenna, wherein the wireless communication is configured touse the at least one antenna to perform wireless communication; andprocessing hardware coupled to the wireless communication circuitry,wherein the processing hardware is configured to operate with thewireless communication circuitry and at least one antenna to: receive,from the source node, a first set of data symbols in a first channel ofthe wireless communications system; transmit, to the destination node,the first set of data symbols in a second channel of the wirelesscommunications system, wherein the destination node is furtherconfigured to receive a second set of data symbols from the source nodein the second channel, wherein the data symbols in the second set ofdata symbols are functions of the data symbols in the first set of datasymbols.
 58. The relay node of claim 57, wherein the first and secondchannels are first and second timeslots of a frame structure of thewireless communications system.
 59. The relay node of claim 57, whereinthe first and second channels are first and second frequencies of aframe structure of the wireless communications system.
 60. The relaynode of claim 57, wherein the first and second channels are first andsecond spreading codes of a frame structure of the wirelesscommunications system.